Following its identification as a black hole candidate, we observed MAXI J1836-194 with the EVLA on 2011 Sep 03 (02:52-03:23 UT; MJD 55807.13). We simultaneously observed at central frequencies of 4.6 and 7.9 GHz, with an observing bandwidth of 128 MHz at each frequency.

We detected a radio source at a position consistent with the Swift X-ray and UVOT localizations (ATel #3613) to within their 90% confidence intervals. Our best radio position (measured at 4.6 GHz) is:

X-ray spectral analyses (ATel #3626) show that the source likely made a transition from the hard intermediate state to the soft intermediate around 2011 Aug 31 (MJD 55804.5). RXTE data taken on on 2011 Sep 02 (11:25-13:20 UT; MJD 55806.52) are well modeled by a disk blackbody (kT ~ 1.2 keV)+ power-law (photon index Γ ~ 1.7) assuming the absorption column density of 2 &times 1021 cm-2 reported in Atel #3613. The total unabsorbed flux (2-10 keV) is ~ 1.1 &times 10-9 erg cm-2 s-1, with ~ 70% of the flux arising from the power-law component. This is consistent with the source still being in transition from the hard to soft state a day prior to the radio observation.

The X-ray and radio data taken together strongly suggest that MAXI is a black hole X-ray binary that has just ejected a jet when transitioning from the hard to the soft state. The strength of the radio emission indicates that MAXI is unlikely to be a neutron star X-ray binary.

The JACPOT XRB collaboration will continue monitoring this outburst with VLBA & EVLA observations, and will be triggering RXTE & Faulkes Telescope monitoring programs.
The first VLBA observations are scheduled for 2011 Sep 4 01:00-05:00 UT.
Further multi-wavelength observations of this source are highly encouraged.

We thank the EVLA & VLBA staff for rapidly scheduling these observations.